Explanation:
Discovered in mid-2005,
Pluto's
small moons were
provisionally designated
S/2005 P1 and S/2005 P2.
They have now been officially
christened Nix and Hydra.
Compared to Pluto and its
large moon Charon, at 2,360 and 1,210 kilometres in diameter
respectively, Nix (inner moon) and Hydra (outer moon) are tiny,
estimated to be only 40 to 160 kilometres across.
Pluto and Charon are bright enough to create
diffraction spikes
in this Hubble Space Telescope image, but
Nix and Hydra are
some 5,000 times fainter than Pluto and
appear only as small points of light.
Still, their new names are appropriate for the distant
Pluto system.
In mythology,
Nix was
the goddess of darkness and night and
the mother of Charon,
while Hydra
was a nine headed monster
and is now orbiting the solar system's ninth planet.
Of course Nix and Hydra also share initials with
the pluto-bound spacecraft
New Horizons.